This invention relates to a method and system for the preparation of specimens for analysis by electron microscopy, and more particularly to a method and system for the milling of specimens using an ion beam.
Ion beam milling systems are used for the preparation of specimens whose internal and surface structures are then analyzed using transmission electron microscopy (TEM) or scanning electron microscopy (SEM). Many techniques have been developed which have included the use of mechanical cutting, grinding, and/or polishing operations in combination with ion beam bombardment. In many instances, such techniques have required that the specimen be repeatedly moved from one apparatus to another (either different preparation devices or observation devices) and/or be moved or realigned while mounted in a cutting, grinding, or milling device.
Transmission electron microscopy is an important technique for studying the detailed microstructure of many materials. Improvements in the design and operation of electron microscopes have generated considerable interest in obtaining atomic resolution images of various materials. However, the preparation of specimens for atomic resolution transmission electron microscopy is demanding, requiring a final specimen which is very thin (i.e.,<50 nm) and free from artifacts. Typically, specimen preparation involves initial slicing, sectioning, trepanning, and/or grinding operations to produce a relatively thin (100-200 μm) disk of approximately 3 mm diameter.
Ion beam milling systems have been used to prepare specimens of various materials including ceramics, semiconductors, metals, and combinations thereof for atomic resolution transmission electron microscopy. In such ion beam milling systems, such as the system disclosed in commonly-assigned U.S. Pat. No. 5,009,743, to Swann, specimens are mounted on holders and placed in the path of one or more ion beams. The ion beams gradually remove atoms from the surface of the specimen until a small perforation is formed in the center of the specimen. Generally, the area immediately around the perforation is then thin enough (i.e.,<50 nm) for atomic resolution analysis by a transmission electron microscope.
Swann et al, U.S. Pat. No. 5,472,566, provides a specimen holder adapted to permit the simultaneous two-sided ion beam milling of a specimen at very low angles of beam incidence, down to 0°, from both sides of the specimen through the use of one or more support arms to secure the specimen. This produces both rapid milling as well as reducing artifacts to provide high quality specimens for transmission electron microscopy analysis.
In such ion beam milling systems, the ion milled specimens have to be removed from the specimen holders and loaded in a TEM holder for imaging. Clearly, loading and unloading of the specimen from the holders increases the potential of damage to fragile specimens. Additionally, if the specimen is not completely electron transparent upon initial TEM examination, it must be returned to the ion mill for further thinning. Multiple milling steps may be required, depending upon thickness requirements at the area of interest in the specimen.
Slope cutting by directing an ion beam at a masking screen located at or near a specimen surface is also known. The masking screen protects a portion of the specimen, while the remainder of the specimen is milled by the ion beam to reveal desired profiles. This technique is capable of providing cross-sectional profiles of layered structures, the surfaces of which can then be studied by scanning electron microscopy. For example, Hauffe, East German published Patent Application No. 201,538, teaches an apparatus and method for preparing specimens for SEM analysis by providing an ion gun inside the specimen chamber of a scanning electron microscope. A mask is positioned adjacent the sample and is used, in conjunction with the ion gun, to remove material from the specimen.
Double slope cutting to produce a very thin film of a specimen for TEM analysis has also been developed. Yoshioka et al, U.S. Pat. No. 5,907,157, describe a method and apparatus for preparing a specimen adapted for electron microscopy which includes an evacuated specimen-processing chamber. A specimen having a surface to be processed is placed inside the processing chamber and a beam-blocking member is placed close to the processed surface so as to block a part of an etching beam. A first etching step is performed by directing the beam at the specimen via the blocking member. Then, the specimen and the blocking member are moved relative to each other to expose another portion of the specimen. A second etching step is then performed by directing the beam at the specimen via the blocking member. As a result, the specimen is thinned and can be observed with an electron microscope. However, the requirement that the specimen and blocking member be moved relative to one another to perform the second etching step complicates the process and introduces the potential for errors as the relative positions of the two must be controlled to within an accuracy on the order of a few hundred nanometers (10−9 meters) or less.
Mitro et al, U.S. Pat. No. 5,922,179, teaches an apparatus and process for etching and coating specimens in a single vacuum chamber, to minimize handling and transfer. The apparatus includes a sealed chamber and a vacuum pump for forming and maintaining a vacuum in the chamber, a first ion gun positioned in the chamber to etch a specimen, a sputtering target in the chamber, and at least one additional ion gun positioned in the chamber to cause material from the target to be directed onto the specimen.
Techniques have also been developed in the preparation of specimens for scanning electron microscopy in which the specimen may be observed by a SEM during preparation thereof. For example, Grunewald, U.S. Pat. No. 5,986,264, teaches an ion beam milling system using two ion guns for use in the preparation of samples to be used in both SEM and TEM analysis. The system uses an SEM as a high resolution imaging device to observe the progress of the ion milling of a sample and to determine when a proper sample thickness has been achieved. Also, as the sample stage can be tilted, the system is also taught to be useful for slope cutting of specimens.
The inadequacy in localized thinning of TEM specimens with broad ion beams, coupled with the ability of focused ion beam (FIB) to micro-machine materials, has led to the development of FIB milling systems. The first FIB based instrument (Precision Ion Milling System (PIMS)), became commercially available in 1984 from Gatan, Inc. Since the introduction of this instrument, several other commercial FIB systems have become available. These machines are capable of both imaging and milling selected areas within TEM and SEM specimens with submicron resolutions. The PIMS system is a gas ion source (argon) instrument, whereas the newer generation FIB systems employ liquid metal (gallium) ion sources having superior milling and imaging capabilities.
Specimens are normally milled in gallium based FIB systems with higher energy ions (compared to the PIMS), which may produce a thick amorphous surface artifact (damaged layer). This damaged layer should be removed to produce high quality specimens. Post-FIB milling in broad ion beam systems for final milling and cleaning of such specimens is a common practice. Specifically, FIB milled specimens have been re-milled and cleaned in broad ion beam machines using the specimen holder described in the above-mentioned U.S. Pat. No. 5,472,566 to improve specimen quality. However, again, this requires moving the specimen among various specimen preparation systems and the final TEM or SEM.
Others have reported the use of a wire shadow technique for TEM specimen preparation. Senz et al, Ultramicroscopy 70 (1997) pp. 23-28, and Langer, Elektronenoptische Untersuchung des Werkstoffverhaltesn in mechanisch belasteten Mikrobauteilen (1996) teach gluing a wire onto a specimen surface and exposing the surface to ion milling by a Gatan Duo-Mill. The non-shadowed areas of the specimen are removed, and a thin area is produced in the wire shadow.
However, a need still exists in this art for simplified methods and techniques for specimen preparation for both SEM and TEM analysis.